Metal founding – Process – Shaping liquid metal against a forming surface
Reexamination Certificate
1999-08-27
2002-05-14
Lin, Kuang Y. (Department: 1722)
Metal founding
Process
Shaping liquid metal against a forming surface
C474S211000, C474S211000
Reexamination Certificate
active
06386269
ABSTRACT:
TECHNICAL FIELD
This invention relates to a manufacturing method for thin-plate magnets ideal for magnetic circuits used in various small motors, actuators, and magnetic sensors, etc. In the present invention, magnets are obtained, by the method of continuously casting a melt having a specific composition containing 6 at % or less of a rare earth element and 15 to 30 at % boron, on a turning cooling roller or rollers, in a prescribed reduced-pressure inactive gas atmosphere, so that the magnets have a crystalline structure wherein, in the state wherein they are cast, 90% or more is actually constituted by an Fe
3
B compound and &agr;-Fe coexisting with a compound phase having an Nd
2
Fe
14
B crystalline structure, and exhibit a fine crystalline structure having for each configuring phase a mean crystal grain diameter of from 10 to 50 nm. The present invention relates to a method of manufacturing thin-plate magnets, immediately from the alloy melt, having a fine crystalline structure with a thickness of 70 to 500 &mgr;m and exhibiting magnetic properties of iHc≧2.5 kOe and Br≧9 kG.
BACKGROUND ART
Even higher performance, smaller size, and lighter weight are now being demanded in home electric appliances, automated office equipment, and electrical fixtures. In this context, design work is being done to maximize the performance-to-weight ratio in the entire magnetic circuits which use permanent magnets. In particular, in the structures of brush-type DC motors that make up the majority of the motors now being produced, permanent magnets having a residual flux density Br of 5 to 7 kG or so are considered ideal, but such cannot be obtained with conventional hard ferrite magnets.
Such magnetic characteristics can be realized in Nd—Fe—B sintered magnets and Nd—Fe—B bonded magnets wherein the main phase is Nd
2
Fe
14
B, for example. However, as they contain 10 to 15 at % of Nd which requires a great number of process steps and large-scale equipment for the metal separation refinement and reduction reactions, their cost becomes much higher than the cost of hard ferrite magnets, and, in view of the performance-to-cost ratio, they are only replacing hard ferrite magnets in some models. Currently, moreover, no inexpensive permanent magnet material which exhibit a Br of 5 kG or higher has yet been found.
Furthermore, in order to achieve smaller and thinner magnetic circuits, thin-plate permanent magnets wherein the thickness of the permanent magnet itself is on the order of 100 to 500 &mgr;m are being sought. However, because of the great difficulty of obtaining bulk material for Nd—Fe—B sintered magnets having a thickness of less than 500 &mgr;m, such can only be fabricated by grinding a plate-form sintered body several mm in thickness or by the method of slicing the bulk material with a wire saw, leading to the problems of high cost and low yield.
Nd—Fe—B bonded magnets are obtained by using a resin to bond together powder having a diameter of 10 to 500 &mgr;m, wherefore it is very difficult to form bonded magnets having a thin-plate thickness of 100 to 300 &mgr;m.
Recently, in the field of Nd—Fe—B magnets, a magnet material has been proposed wherein an Fe
3
B compound is made the main phase with an Nd
4
Fe
77
B
19
(at %) neighboring composition (R. Coehoorn et al., J. de Phys, C8, 1988, pages 669, 670). The details of this technology are disclosed in U.S. Pat. No. 4,935,074.
Even earlier, in U.S. Pat. No. 4,402,770, Koon proposed a method for manufacturing a permanent magnet made up of fine crystals wherein an La—R—B—Fe amorphous alloy containing La as a mandatory element is subjected to a crystallizing heat treatment.
More recently, Richter et al. have reported producing amorphous flakes by spraying an Nd—Fe—B—V—Si alloy melt containing 3.8 to 3.9 at % of Nd onto a turning copper roller, heat treating these flakes at a temperature of 700° C., and thus obtaining thin pieces having hard magnetic properties, as disclosed in EP Patent Application 558691B1. These permanent magnet materials obtained by subjecting amorphous flakes having a thickness of 20-60 &mgr;m to a crystallizing heat treatment have a metastable structure containing a crystalline aggregate structure wherein a soft magnetic Fe
3
B phase and a hard magnetic R
2
Fe
14
B phase are mixed.
The permanent magnet materials noted above exhibit a Br of around 10 kG and an iHc of 2 to 3 kOe, wherein expensive Nd is contained in a low concentration of 4 at % or so, wherefore the raw materials mixed in are less expensive than for Nd—Fe—B magnets wherein the main phase is Nd
2
Fe
14
B.
In the permanent magnet materials described above, however, the rapid solidification conditions are limited because making the raw materials so mixed in to an amorphous alloy is a mandatory condition, and, at the same time, the heat treatment requirement for obtaining a hard magnetic material are narrowly limited. Hence these are not practical in terms of industrial production, and thus cannot be provided as inexpensive replacement products for hard ferrite magnets. Furthermore, such permanent magnet materials are obtained by subjecting amorphous flakes having a thickness of 20 to 60 &mgr;m to a crystallizing heat treatment, wherefore it is not possible to obtain permanent magnets having a thickness of 70-500 &mgr;m required for thin-plate magnets.
Meanwhile, U.S. Pat. No. 4,802,931 discloses a rapidly solidified Nd—Fe—B magnet material consisting of a structure formed of a crystalline substance exhibiting hard magnetism that is directly obtained by rapidly solidifying an alloy melt on a roller rotating with a circumferential speed of about 20 m/s. However, the rapidly solidified alloy flakes obtained under these conditions have a thickness of around 30 &mgr;m, wherefore, although they can be ground to a powder having a grain diameter of between 10 and 500 &mgr;m or so and thus used in the bonded magnets described earlier, they cannot be used in thin-plate magnets.
DISCLOSURE OF INVENTION
An object of the present invention is to resolve the problems noted above in Nd—Fe—B magnets containing 6 at % or less of a rare earth element and exhibiting fine crystallization. Another object thereof is to obtain, by casting, magnets exhibiting a performance-to-cost ratio comparable to hard ferrite magnets and exhibiting an intrinsic coercive force iHc of 2.5 kOe or greater and a residual magnetic flux density Br of 9 kG or greater. Yet another object thereof is to provide a method for manufacturing thin-plate magnets having a fine crystalline structure and thickness of 70 to 500 &mgr;m wherewith it is possible to make magnetic circuits smaller and thinner.
The inventors previously disclosed (in Japanese Patent Application No. H8-355015/1996) how to obtain a fine crystalline permanent magnet exhibiting hard magnetic properties (iHc≧2 kOe and Br≧10 kG) directly from alloy melts, by a manufacturing method wherein alloy melts of a low-rare-earth Nd—Fe—B ternary structure containing 6 at % or less of Nd and 15 at % to 30 at % of boron are continuously cast on a cooling roller turning with a roller circumferential speed of 2 to 10 m/s in a specific reduced-pressure inert or inactive gas atmosphere. There is a problem in this method of manufacturing Nd—Fe—B ternary magnets, however, in that the roller circumference speed range must be narrowly limited to obtain the hard magnetism. In addition, in these Nd—Fe—B ternary magnets, the best coercive force obtainable is on the order of 2 to 3 kOe. As a consequence, not only is the thermal demagnetization great, but it is necessary also to raise the operating point of the magnets as high as possible, whereupon problems arise due to limitations in terms of magnet shape and utilization environment.
The inventors thereupon conducted multifaceted research on the problems involved in manufacturing Nd—Fe—B fine crystalline permanent magnets containing low rare earths wherein soft magnetic phases and hard magnetic phases are present together in a nano-mater size scale. As a result of this research, the inventors found that t
Hirosawa Satoshi
Kanekiyo Hirokazu
Dykema Gossett PLLC
Lin Kuang Y.
Sumitomo Special Metals Co. Ltd.
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